Why can't I exercise on my back while pregnant?

Q: Why can't I exercise on my back while pregnant?

A: The weight of your uterus increases throughout pregnancy, so if you were to spend time lying flat on your back, that extra weight might compress the vena cava, the large blood vessel that runs along your spine and carries blood to the heart. Compression of this vein may cause you to become dizzy, lightheaded, nauseated or sweaty, and it may briefly reduce blood flow to your baby. For this reason, you should avoid the supine, or flat-on-the-back, position after about the third month (though some supine exercises can be modified by doing them on your side or by propping up your back and shoulders with pillows). You should also speak with your doctor to be sure you are following safe practices while exercising.